My U.S. Pat. No. 5,017,758 discloses a contact heater for attachment directly to a fluid reservoir such as the oil pan or the like with the heater utilizing the housing and the fluid contained in the housing as a heat sink to keep the contact heater from overheating.
The present invention comprises an improvement to contact heaters that includes a method of making a contact heater wherein the heating element and an electrical ground member are encapsulated in an envelope to prevent inadvertent shock to the user.
One of the difficulties in making an electric contact heater and particularly a low profile, aftermarket, contact heater is the need to form a heater that has a high watt density and can rapidly conduct the heat to the housing of a fluid reservoir while at the same time providing a contact heater that is sufficiently compact so as not to interfere with the operation of the equipment that it is secured to. One such type of heater, which is shown and described in my U.S. Pat. No. 5,017,758, uses layers of electrically insulating material sandwiched around a heating element. The layers of material perform a dual function in that they protect one from directly contacting the heating element while at the same time they rapidly conduct heat from the heating element to a housing of a fluid reservoir which is to be heated so that fluid contained within the fluid reservoir can be heated to a proper temperature. The use of a foil ground member on the exterior of the heater provides an electrical ground as well as a heat conductor. In order to provide an electrical ground member on the opposite or exposed side of the heating element it is proposed to include a similar foil ground member on the heater. However, since the exposed side can come into contact with a person it is proposed that the ground member on the opposite side of the heating element be incorporated into the envelope layers sealing and protecting the heating element.
Although the materials used to envelope the heater are gas permeable, it has been found that the process of assembly can cause the materials forming the envelop to blister if the materials are vulcanized or otherwise heat sealed to each other when a ground member is incorporated into the envelope. In addition, once assembled abnormal operational conditions can also cause the heater to blister thereby rendering the heater unsuitable for use.
In the preferred embodiment the heater relies on the heat capacity of the reservoir that it is attached to the heater to form a heat sink that quickly and continually dissipates heat so that the heater does not overheat; however, the present invention is also suitable for other heaters that might exceed a temperature that causes the heater to overheat and breakdown, which is referred to as a critical temperature. The critical temperature or heater breakdown temperature is well above the conventional temperature operating range of the heater and is primarily determined by the materials surrounding the heating element. That is, while the heating element usually does not breakdown if the heater exceeds the critical temperature the materials forming the sealed envelope around the heating element can blister and cause failure of the heater.
It would be desirable to extend the critical temperature in an internal grounded heater so that an inadvertent overheating of an internal grounded heater would not result in destruction of the layers of material forming the heater. The present invention provides an internal grounded heater, and more specifically a double grounded heater, wherein both sides of the electrical heater have a ground member proximate thereto. The heater of the present invention can operate at an elevated temperature that would normally produce heater breakdown. In addition one can heat seal the envelope layers of the heater without causing destruction of the heater.
The prior art method of forming a contact heater utilizes layers or flexible sheets of an electrical insulator such as sheets of silicone which are sandwiched around a heating element. To prevent accidental electrical shock in the event of a heater failure an external ground members extends along at least one face of the layers of electrical insulation located on the heating element. This type of heater is referred to as a single grounded heater with external envelope ground since the ground element is not encapsulated in a sealed envelope.
In some applications it may be necessary to envelope and seal the heater within the envelope and at the same time extend an internal ground member along both sides of the electrical insulator to minimize any opportunity for accidental electrical shock to a person or persons proximate the heater. This type of heater is a double grounded heater with an internal ground since there are electrical ground members on both sides of the heating element with one of the ground members located in the sealed envelope surrounding the heating element. In another embodiment only an internal ground member within the sealed envelope is provided. This type of heater is a single grounded heater with an internal envelope ground since the only ground member is internal to the envelope surrounding the heating element.
Unfortunately, the extending of an internal electrical ground member along the heating element and then encapsulating the electrical ground member in the sealable envelope layers can result in difficulties in manufacture of the heater as well as premature failure of the heater. That is, the enveloping of the heater and ground member in a sealed envelope can cause blistering of the envelope during the heat sealing process. In addition, if the enveloping process did not produce blistering, if one inadvertently overheats the heater the layers of material can blister or bubbles therein thereby rendering the heater unusable. While envelope formation failure is an assembly process failure the operational failure occurrences do not usually incur unless the heater is not properly secured to a heat sink. It would be preferable if such heater failure due to accidental overheating could also be prevented even though normal operation precludes the heater from reaching a critical temperature where such a failure can occur.
The present invention provides an apparatus and method of making a self adhesive contact heater with a double ground and wherein the ground member in is in envelope in the heater is gas permeable to provide a heater that remains intact and operable even though the heater encounters a temporary overheated condition as well as allows one to heat seal the heater envelope without destroying the heater.